Automobiles are normally equipped with door locks that should only be released with a correct key, and also with ignition locks so that the engine should only be started with a correct ignition key. These locking devices help protect automobiles against theft. To help ensure against theft, theft prevention mechanisms have been proposed and are in practical use.
An example of such a device can be found in "Automobile Technology Case Study"/issue No. 95603 (Japan Automobile Manufacturers Association, Intellectual Property Council: Dec. 1, 1995), where there is disclosed an anti-theft system for vehicles which is interlocked with an ignition key in such a way that the engine is prohibited from being started unless operated with a correct ignition key.
In this anti-theft system for vehicles, the ignition key has a built-in transponder for transmitting a personal identification (ID) code. This transponder receives power from electromagnetic radiation transmitted from the antenna provided on the key cylinder, and transmits an ID code. The antenna of the key cylinder is connected to the immobilizer ECU, and this immobilizer ECU decides whether the received ID code is correct or not. The immobilizer ECU is connected to the engine ECU for control of the ignition and fuel injection in the engine, and the immobilizer ECU notifies the engine ECU whether or not a correct ID code was received. If the immobilizer ECU did not receive the correct ID code, the engine ECU prohibits fuel injection and ignition in the engine, thereby halting the engine. Thus, if someone tries to start the engine without using a correct key, starting of the engine can be prevented so that the vehicle cannot run.
Note that in this case the engine ECU has already started the engine in the usual manner by ignition by manipulation of the ignition key, and that the device causes the engine to stop in response to a signal from the immobilizer ECU. In the early stage of the engine start operation there are great variations in the source voltage because the starter motor is rotating. For this reason, communication occurs between the immobilizer ECU and the engine ECU after the engine speed has increased to greater than 500 rpm.
Specifically, when the number of engine revolutions has exceeded 500 rpm, the engine ECU creates a predetermined rolling code A by random number generation, and sends the code to the immobilizer ECU. The immobilizer ECU, when it has recognized the correct key, generates a rolling code B by random number generation, and returns the rolling code B to the engine ECU. From the code B, the engine ECU confirms that the correct key has been used, and finishes the key confirmation process so that the ordinary engine operation is continued.
When a correct rolling code B is not returned within a predetermined period of time (a few seconds, for example), the engine ECU stops ignition and fuel injection. In other words, if the key inserted is not the correct key, the engine is stopped.
In a system as in the above mentioned example, when the immobilizer ECU decides that a theft has occurred as a result of code collation, the immobilizer ECU sets the communication line from the immobilizer ECU to the engine ECU to the L level. When detecting the L level of the communication line, the engine ECU stops the engine. Therefore, in this system, when a decision is made that this is a theft, the engine can be stopped at an early stage.
In the above system, when the communication line from the immobilizer ECU to the engine ECU is fixed at the H level, the engine does not stop for five seconds from the instant the number of engine revolutions reaches 500 rpm (for a period until it is confirmed that no rolling code has been returned). Consequently, the vehicle moves to some extent.
As the key insertion detecting switches are known which detect key insertion, the manipulation of the key can be detected at an early stage and it is therefore considered possible to finish key collation before the starter motor begins to rotate.
However, if the key insertion detecting switch is out of order, it is impossible to start the engine when this key insertion detecting switch is used to immediately collate the key as if the key insertion detecting switch is out of order, the immobilizer ECU does not start the collating action and a coincidence signal cannot be obtained. Because of this, the engine is stopped whenever the key is inserted.
In Japanese Patent Laid-Open Publication No. Hei 7-307983, it has been proposed to prolong the decision time according to a cranking state, but the problem in this case is that only time prolongation is done, so that it takes too long a time before a permission to start the engine is given.
The present invention has been made to solve the above problem, and has as its object to provide an anti-theft system for vehicles, which is capable of making an efficient decision about the adequacy of the key, even when the key insertion detecting switch is out of order.